1. Field of the Invention
This invention relates to motor rotors, and, more particularly, to a motor rotor that reduces noises generated by a cogging torque and a motor having the motor rotor.
2. Description of Related Art
Electrical motors are applied to numerous kinds of products. In general, a motor forms an inductive magnetic field by using electrical current. The inductive magnetic field interacts with an inner permanent magnet to generate rotational movement. In addition to providing the rotational movement, the motor may also output rotational mechanical energy that may be converted into a linear or vibrating movement by certain mechanisms.
FIG. 1A (PRIOR ART) provides an oblique view of an outer rotor motor according to the prior art. Referring to FIG. 1A (PRIOR ART), the outer rotor motor comprises a stator 11 installed in the central region and a rotor 12 surrounding the outer circumference of the stator 11. The stator 11 is a stationary part. The stator 11 comprises field coils 111, and generates a magnetic field by inputting currents into the field coils 111. The rotor 12 includes an annular body 121 and a plurality of permanent magnets 122 installed on an inner lateral surface of the annular body 121. The permanent magnets 122 provides another magnetic field. The rotor 12 generates rotational movement according to interaction of the two magnetic fields.
FIG. 1B (PRIOR ART) provides a top view of the outer rotor motor shown in FIG. 1A (PRIOR ART). Referring to FIG. 1B (PRIOR ART), the permanent magnets 122 are arranged on the inner lateral surface 123 of the annular body 121 spaced at an equal interval.
However, the equal interval arrangement of the permanent magnets results in a cogging effect and cogging torque while a motor is rotating. Since the permanent magnets are arranged at an equal interval, the cogging torque has a constant frequency and amplitude (as shown in FIG. 1C (PRIOR ART)). If the frequency of the cogging torque is close to a resonance frequency of the motor or something nearby or is within people's hearing range, people will be annoyed by the noises, and the motor is deemed to have poor quality.
Therefore, solving the drawbacks of the prior art motor to reduce the noise generated while the motor is operating is an important goal in the art.